This invention relates to a dual worm extruder which has a worm housing and an insert which is made of a wear resistant metal and which is accommodated in the housing. The wear resistant insert has two parallel-extending overlapping bores which, when viewed in cross section, form an open "8". Throughout its length, the insert has strictly constant dimensions to ensure an accurate fit in a complemental opening in the worm housing.
By "dual worm extruder" there are meant all conventional extruders or injection apparatus which are used in a great variety of fields for making plastic or rubber components and which, compared to extruders having only a single conveying worm have the advantage that the material is discharged in a more uniform manner. The housing for such a dual worm is so configured that the opening itself, when viewed in cross section, has the form of an open "8", that is, the bores for the two worms are open towards one another where they overlap. The two bores may be cylindrical throughout their length or may have a conical configuration.
Since the worms rotate in the worm housing and the clearance between the respective worm and the face of the associated worm housing has to be very small in order to build up the necessary pressure for injecting the material, the inner face of the worm housing is exposed to a continuous wear. Consequently, such inner face has to be of a material which is particularly wear resistant. For making a wear resistant surface in an extruder housing several methods have been known.
In principle, it is feasible to make the entire extruder housing of a particularly wear resistant material, such as high-grade steel. Because such material is very expensive, this solution, although technically highly satisfactory, would not be economically feasible. For this reason, the housing of the extruder is usually made of a significantly weaker (and less expensive) material and the bore faces are coated by a wear resistant layer.
In coating extruder housings with a wear resistant material, it is known, for example, to harden the inner face of the extruder housing by nitration which is effected under relatively low temperatures of approximately 500.degree. C. With this method layers of maximum 0.7 mm thickness are obtained which is insufficient for many applications. In case greater layer thicknesses are desired with this process, higher temperatures have to be used which, however, can easily lead to deformations of the housing, resulting in the loss of its linear characteristics. Should this occur, the housing subsequently has to be straightened which involves the disadvantage that mechanical stresses will be "built" into the housing; these stresses may become free during the service of the extruder, again leading to a deformation of the housing. Such a deformed extruder can no longer be driven. It is a further disadvantage of this hardening process that the hardness obtained for the wear resistant layer is not sufficiently high for many applications.
A further method of providing a wear resistant layer on the inner face of the extruder housing involves the introduction into the housing, of a premade shaped body of wear resistant metal. This solution--with which the invention is concerned--is of particular advantage in dual worm extruders which have an inner housing surface of relatively complex configuration.
A dual worm extruder having a wear resistant insert is disclosed, for example, in German Accepted Published Patent Application (Auslegeschrift) No. 2,423,785 to which corresponds U.S. Pat. No. 4,028,027. The wear resistant insert disclosed therein has a periphery formed of either solely convex surfaces or of convex and planar surfaces. Such a wear resistant insert must be manufactured with great precision in case a throughout accurate fit and surface contact with the worm housing is desired. In particular, the transitions from the convex surfaces to the linear surface portions give rise to substantial manufacturing problems. The worm housing opening through which the wear resistant insert is to be introduced has to be made with high precision as well. Even the smallest deviations from the required dimensions may lead to the disadvantageous result that the wear resistant insert can simply not be introduced into the worm housing or that insufficient surface contact between the worm housing and the wear resistant insert is obtained. This deficiency leads to an unsatisfactory heat exchange.
To remedy the above-discussed disadvantage, in German Accepted Published Patent Application (Auslegeschrift) No. 2,558,611 there is disclosed a method according to which the outer dimensions of the wear resistant insert are intentionally reduced relative to the inner dimensions of the opening in the worm housing. The annular clearance defined between the outer face of the wear resistant insert and the inner face of the worm housing is filled with a brushable, hardening material. Thus, in an arrangement arrived at with such a process, there is no direct contact between the wear resistant insert and the worm housing; this again leads to an unsatisfactory heat removal from the worm cavity.